DEVELOPMENT OF A GROUNDWATER MODEL FOR INDEPENDENCE VALLEY, NORTHERN ELKO COUNTY, NEVADA

The Tuscarora mining district in northern Elko County, Nevada has been a focus of gold and silver mining since 1871. Since then, placer and lode mining have been conducted sporadically up to 1990, when the Tuscarora open pit mine closed. The area is still of interest and precious metal exploration continues to the present. For this reason a groundwater model is being developed for the Tuscarora vicinity in anticipation of future dewatering activities. Spatial information from a single, low resolution gravity survey and existing geological maps of the area were used to set up the model dimensions and layer thicknesses. The model is designed with three layers with horizontal boundaries of approximately 150 and 450 meters depth below surface. In the Independence Valley area, these figures represent the general depth limits of agricultural pumping and mine dewatering, respectively. Because of the considerable vertical to horizontal anisotropy common to many northeast Nevada basin-fill aquifers, these breaks are useful to isolate distinct groundwater use zones, as shallow and deep pumping often do not interfere. This isolation of shallow and deep aquifers is seen clearly in mine dewatering currently being conductded in the central Carlin Trend mining district in northeast Nevada. The water table aquifer in the Tuscarora area is shallow (about 10 to 20 meters depth below surface near the town), as evidenced by the lake in the Dexter open pit mine, inactive since 1990. Other water level information exists in the form of exploration drilling records, some of which tap deeper confined aquifers, and local water wells. To this date there are no known pumping tests of any of the area aquifers. Historic stream gage data is available from several locations in the valley and the information will be used for initial estimates of shallow hydraulic conductivities. Additional seepage runs will be conducted for supplemental information. This model will enable us to estimate possible impacts from mine dewatering on the local community, wildlife, and riparian vegetation. In the event of mine dewatering, the drawdown information will be used to refine the model and improve its predictive capabilities.